JPH0790530A - Production of surface treated steel material excellent in blacking resistance - Google Patents

Abstract

PURPOSE:To provide a producing method of a surface treated steel material excellent in blacking resistance free from the change of the surface appearance in the storage. CONSTITUTION:An Fe-Ai intermetallic compound layer of <=1mum thickness is formed on the boundary between the steel material and a plated layer by dipping the steel material into a molten metal composed of 0.1-20wt.% Al and the balance substantially zinc, after dipped again in a molten metal composed of <=0.1wt.% Al and the balance substantially zinc to form a plated layer, the steel material is chromate-treated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a surface-treated steel material which is excellent in blackening resistance and whose surface appearance does not change during storage. The surface-treated steel material according to the present invention can be suitably used for a wide range of applications such as automobiles, building materials, and home appliances, and the shape of the steel material used as the base material is a plate shape, a rod shape, a corrugated plate shape, The present invention will be described below by exemplifying a cold-rolled steel sheet as a representative, regardless of whether it is tubular or the like.

[0002]

2. Description of the Related Art Conventionally, it is widely known that hot dip galvanizing is applied to the surface of a steel sheet to improve corrosion resistance.
Even today, hot-dip galvanized steel sheets are used for a wide range of purposes. In addition, in recent years, in addition to corrosion resistance, it has been required to improve the surface appearance such as uniformity and flatness, and in order to meet this requirement, after plating, before the zinc solidifies, it becomes Zero spangle treatment for spraying water droplets to make zinc crystals fine and skin pass rolling treatment after cooling are performed.

On the other hand, conventionally, a chromate treatment has been carried out for the purpose of preventing white rust from occurring on a hot-dip galvanized steel sheet and improving the adhesion of a coating film. However,
In the hot-dip galvanized steel sheet subjected to the chromate treatment, a so-called black discoloration phenomenon in which the surface of the steel sheet is gray-black is often recognized during storage in a warehouse after manufacturing or during construction. Furthermore, this blackening phenomenon is likely to occur in a state of being packaged with a coil or being stacked with a cutting plate, and occurs in a partial or patchy manner, which significantly impairs the appearance.

The cause of such a blackening phenomenon has not been clarified yet, but it is considered as follows. Since the hot dip galvanized steel sheet is passed at high temperature after plating, an oxide film mainly containing zinc is formed on the surface thereof. This oxide film is partially removed by skin pass rolling, etching during chromate treatment, etc., and a new surface of zinc is exposed at that part. In addition to zinc, aluminum that is added to suppress the formation of a Zn—Fe alloy layer is present on the new surface. When a chromate film is formed, the supply of oxygen to the plating layer under the film is suppressed, and in such an environment, zinc is associated with preferential oxidation of aluminum, and zinc is an oxygen-deficient non-stoichiometric oxide. Becomes It is estimated that this non-stoichiometric zinc oxide is the cause of blackening.

By the way, as a method for preventing blackening,
After plating and skin pass rolling, Co, Ni, Fe or the like is deposited on the plating surface by dipping or spraying, and then a chromate film is formed (Japanese Patent Application No. 59-1).
77381), after plating and before the metal solidifies, Co
And a method of forming an oxide film by spraying the treatment liquid containing the cation on the surface (Japanese Patent Application No. 62-50474). Among these, the method of precipitating a metal such as Co, Ni, or Fe on the surface before chromate treatment is difficult to strictly control the adhesion amount of those metals in accordance with the line speed of the hot dip coating line, In some cases, it may cause deterioration of corrosion resistance and poor appearance.
Further, when the washing with water after the metal precipitation treatment is insufficient, impurities are brought into the chromate treatment step, which may deteriorate the quality of the chromate treatment itself.

Next, the method of spraying the Co-containing liquid before the solidification of the plating may destroy the oxide film when performing skin pass rolling after such treatment, which may reduce the effect. Furthermore, it is not easy to uniformly form a Co oxide film, and there is a concern that the appearance, corrosion resistance, and adhesiveness may deteriorate. In addition, all of the above methods are intended to improve the blackening resistance by the treatment after plating, but since it is not possible to remove the aluminum on the plating surface, which has the most influence on the black color change, it is basically black. The current situation is that changes cannot be resolved and their effects are not sufficient.

Conventionally, when performing hot dip galvanizing,
0.1% or more of aluminum is added to the plating bath. If aluminum is not added, a Zn—Fe alloy layer having a high Fe content grows thick at the interface between the steel sheet and the plating layer, resulting in a marked deterioration in workability. Therefore, as a general-purpose material, in order to enable minimum processing, 0.1% or more of aluminum is added to the plating bath to form an extremely thin Fe-Al alloy layer at the interface with the steel sheet. A general method is to suppress the formation of Zn—Fe alloy by using this layer as a barrier for Fe diffusion. Therefore, according to the conventionally known method, aluminum in the plating layer cannot be reduced, and as a result, it is difficult to improve the blackening resistance. Further, a method for reducing aluminum in the plating layer to improve blackening resistance has not been known yet.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the problem that the surface of a hot-dip galvanized steel sheet that has been subjected to chromate treatment is blackened when it is stored. An object of the present invention is to provide a method for producing a surface-treated steel material excellent in modification.

[0009]

A method for producing a surface-treated steel material excellent in blackening resistance according to the present invention is made of aluminum 0.1.
Steel material is immersed in a molten metal (hereinafter, sometimes referred to as the first bath) consisting essentially of -20% by weight and the balance essentially zinc, and Fe-having a thickness of 1 μm or less is formed at the interface between the steel material and the plating layer.
An Al intermetallic compound layer is formed, and aluminum is again formed.
It is characterized in that it is dipped in a molten metal (hereinafter sometimes referred to as a second bath) containing less than 0.1% by weight and the balance being essentially zinc to form a plating layer and then chromate treatment. And

In the above, the material steel material subjected to the surface treatment is typically a cold rolled steel sheet cleaned by a conventionally known method. Cleaning methods for steel sheets include degreasing, fluxing, gas reduction, etc.
The method is not particularly limited as long as it is a method capable of removing the oxide film and obtaining an active surface, and may be appropriately performed according to a conventionally known method.

In the present invention, the composition of the first bath is 0.1 to 20% by weight of aluminum and the balance is essentially zinc. When the aluminum content is less than 0.1%,
The reaction between iron and zinc promotes the growth of a Zn-Fe alloy layer having a high Fe content, while the aluminum content is 2%.
When it exceeds 0% by weight, the growth of the Fe-Al intermetallic compound becomes excessive and the thickness thereof exceeds 1 μm, which both deteriorates the workability of the plated steel sheet remarkably. Therefore, the aluminum content in the first bath must be in the range of 0.1 to 20%, and plating in this range suppresses the diffusion of Fe from the steel sheet into the plating bath. Since a sufficient Fe-Al intermetallic compound is formed and the thickness thereof is 1 μm or less, the workability of the plated steel sheet does not deteriorate. In addition to aluminum and zinc, the first bath may contain inevitable impurities such as iron, and additional third elements such as lead and antimony. These third elements are not particularly problematic in the step of immersing the steel sheet in the first bath to form the Fe-Al intermetallic compound layer, but in the step of immersing the steel sheet in the second bath, When it is mixed in the steel, it may affect the corrosion resistance of the plated steel sheet, so it is desirable to keep it to 0.1% by weight or less.

According to the invention, the steel sheet is immersed in the first bath,
After forming the Fe-Al intermetallic compound layer, the steel plate was pulled up from the first bath and immersed in the second bath to obtain F.
On top of the e-Al intermetallic compound layer is formed a coating layer consisting of less than 0.1% by weight of aluminum and the balance essentially zinc. The composition of the second bath consists of less than 0.1 weight of aluminum and the balance essentially zinc. According to the present invention, in order to improve the blackening resistance of the surface-treated steel material, it is necessary that the content of aluminum in the second bath is less than 0.1% by weight. Further, in the present invention, since the Fe-Al intermetallic compound layer serving as the barrier layer against the diffusion of iron is formed before the dipping in the second bath, the content of aluminum in the second bath is basically set. The smaller is, the more the blackening resistance is improved.
Therefore, it is preferable that the content of aluminum in the second bath is as small as possible.

In the method of the present invention, when the steel sheet is dipped in the first bath and pulled up, a coating layer corresponding to the composition of the first bath is formed on the Fe-Al intermetallic compound layer. This coating layer elutes when immersed in the second bath and instead,
A coating layer corresponding to the second bath composition is formed. Therefore, it is not particularly necessary to remove the coating layer corresponding to the first composition formed in the first bath before dipping in the second bath, however, the steel sheet is dipped in the first bath and pulled up. As it is in the second bath,
When dipping, the first bath components may be mixed in the second bath, which may complicate the composition control of the second bath, so if possible, before dipping the steel sheet in the second bath, use the first bath. It is desirable to remove the formed coating layer corresponding to the first bath component from the steel sheet. As a removal method therefor, a gas restriction method,
Mechanical grinding, chemical etching, etc. can be adopted, and other methods can also be adopted, in particular,
It is not limited.

In the present invention, the plating conditions such as the plating bath temperature and the plate temperature are not particularly limited in both cases of dipping in the first bath and dipping in the second bath, and they are conventionally known. You can follow the usual methods. In the present invention, it is possible to carry out skin pass rolling after plating to adjust the surface texture, and even if such skin pass rolling is carried out, the blackening resistance of the steel material is not adversely affected.

The present invention improves the blackening resistance by reducing the aluminum content in the plating layer, and it is not necessary to perform a special pretreatment before the chromate treatment. It can be performed according to the usual method. For chromate treatment,
Although coating type, reactive type, and electrolytic type are known, any of these types of chromate treatments can be adopted in the method of the present invention, and may be appropriately selected as necessary. Also, the chromate treatment conditions are not limited at all, and the conventionally known normal conditions may be adopted.

The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0017]

EXAMPLES In each of the following examples and comparative examples, as a raw steel sheet, an aluminum-killed cold-rolled steel sheet having a thickness of 0.7 mm, which was cleaned according to a conventionally known method, was used. It processed by the batch type hot dip plating apparatus under the conditions shown in Table 1, and obtained the surface-treated steel plate.

[0018]

[Table 1]

The sample obtained in this manner is used as a rolling reduction.
Skin pass rolling at 0.5%, then dipping in a commercially available chromate treatment liquid, pulling up, ringer roll squeezing, chromate treatment, drying at 60 ° C. for 10 minutes, and then subjecting to performance evaluation test did. The method of the performance evaluation test is as follows. Black Decoloration Resistance Test The chromate treated material was stored under conditions of a temperature of 50 ° C. and a relative humidity of 98% for 168 hours, and the degree of black change on the plating surface was visually observed and evaluated. Workability test The plated steel sheet was subjected to close contact bending, the occurrence of cracks in the worked part was observed, and the degree of cracking was evaluated.

Table 2 shows the test evaluation results of the respective examples and comparative examples.

[0021]

[Table 2]

In Table 2, the evaluation criteria for blackening resistance are as follows:
Indicates no black discoloration area, ○ indicates black discoloration area, but very few
Indicates that the black discoloration area is medium, and ○ indicates that the black discoloration area is large. In addition, the workability evaluation criteria were evaluated as ◯ when the number of cracks in the processed part was small, Δ when the size was moderate, and x when the size was large. As is clear from Table 2, the cold-rolled steel sheets surface-treated according to Examples 1 to 17 according to the method of the present invention are excellent in both blackening resistance and workability. However, the cold-rolled steel sheet surface-treated in Comparative Examples 1 to 6 in which the aluminum content in the first bath or the second bath does not satisfy the conditions specified in the present invention, and the conventional method of treating in the first bath Comparative Examples 7 to 9
The cold-rolled steel sheets according to No. 1 are inferior in either blackening resistance or workability.

[0023]

As described above, according to the method of the present invention,
A hot-dip galvanized steel sheet excellent in blackening resistance can be obtained.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Tomio Kajita 2222 Address 1 Ikeda, Ikeda, Ikeda, Onoue-cho, Kakogawa-shi, Hyogo Prefecture Kakogawa Research Area, Kobe Steel Co., Ltd.

Claims (1)

[Claims] 1. A Fe-Al intermetallic compound having a thickness of 1 μm or less at the interface between a steel material and a plating layer by immersing the steel material in a molten metal consisting of 0.1 to 20% by weight of aluminum and the balance being essentially zinc. Formed a layer, again 0.1% by weight of aluminum
A method for producing a surface-treated steel material excellent in blackening resistance, which comprises immersing in a molten metal whose lower portion and the remainder are essentially zinc to form a plating layer and then performing a chromate treatment.